1. Field of the Invention
The present invention relates to an optical recording medium and more particularly to an optical recording medium of a groove recording type in which record is made in a recording layer formed on a groove.
2. Description of Related Art
Optical disks in general (including those of a write once type and a rewritable type) are as shown in FIG. 5 provided with a spiral groove (pre-groove) 22 constituting a track made in the surface of a glass substrate 21 and with a recording layer 23 formed on the entire surface including the groove 22. Referring to FIG. 5, reference numeral 24 denotes a land surface and 25 denotes a groove surface. The optical disk has so narrow a track pitch P as 1.6 .mu.m that it has ten times or above as high a recording density as that of magnetic disks (with a track pitch of 10 .mu.m) which are frequently used today as a recording medium. Further, the optical disk having an access time of 50 msec or so provides an advantage to allow an extremely fast access thereto, and since it allows recording and reproduction of signals thereon in a noncontact manner, it has strong resistance to dust, scratch, or the like. Hence it presents an outstanding feature that the signal reproduced therefrom suffers no deterioration by mechanical causes even if it is subjected to many times of reproduction.
As the methods for recording signals in such optical disks, there are a land recording method and a groove recording method. An optical disk conventionally used for the land recording method has a width Wl of the land surface 24 of 1.0 .mu.m and a width W.sub.Gr of the groove surface 25 of 0.6 .mu.m, including the sloped surfaces 26l and 26r on the left and right, as shown in FIG. 6A. Thus, it has the groove surface 25 narrower than the land surface 24 and called a narrow-groove type. On the other hand, an optical disk conventionally used for the groove recording method has a width Wl of the land surface 24 of 0.6 .mu.m and a width W.sub.Gr of the groove surface 25 of 1.0 .mu.m as shown in FIG. 6A. Thus, it has the groove surface 25 wider than the land surface 24 and called a wide-groove type.
As a measure for evaluating noise in an optical disk, CN (carrier to noise) ratio is generally used. The CN ratio is the ratio of a carrier level to a noise level obtained as the result of a frequency spectrum analysis conducted, having data of a fixed frequency and a duty cycle of 50% recorded on an optical disk, in a 30-kHz resolution bandwidth of the entire frequency band of the reproduced waveform (refer to FIG. 7).
To improve the CN ratio of an optical disk, for example, of the groove recording type out of those for the above mentioned two recording types, it is required, when the optical disk is of a reflection type, to increase the quantity of reflected light of a laser beam l thrown on the groove surface 25 from the bottom surface of the glass substrate and bring it as close to the quantity of reflected light from the mirror surface (not shown) as possible. Representing the quantity of reflected beam from the groove surface 25 by I.sub.Gr and the quantity of reflected beam from the mirror surface by I.sub.0, I.sub.Gr /I.sub.0 =0.7-0.8 was conventionally the achievable maximum value and, then, the CN ratio was around 48 dB.